Divers are being reminded to raise red flags while they’re underwater following an incident in Lanikai over the weekend that left one man dead and another injured.

Hawaii administrative rules say divers are required to put up distinctive diver’s flags that are red with a white diagonal strip running across them, said Ed Underwood, administrator for the state Division of Boating and Ocean Recreation. The flags must be at least 12 by 12 inches, larger if they are on vessels 16 feet in length or bigger.

The Honolulu Medical Examiner’s Office said Monday that the cause in the death of Kailua resident Sri Shim, 59, on Saturday was the result of “blunt force and propeller injury.” The ME’s office characterized the death as an accident.

Shim’s stepson, 25, also sustained serious injury after being struck by a passing boat, Emergency Medical Services personnel said.

Underwood told reporters that divers are supposed to put up the flags before they submerge themselves in water.

Fire officials said the two men were free-diving in waters off Lanikai Beach at about 1 p.m. when the incident occurred. A diver’s flag was found in the general vicinity of the accident but was later determined to belong to a separate person who was diving in the area, fire officials said.

A search by fire and ocean safety personnel was initiated after Shim’s kayak was located approximately 600 feet offshore. His body was recovered two hours later about 200 feet away from the boat.

The incident is being investigated by the state Department of Land Natural Resources’ Division of Conservation and Resources Enforcement. The division declined to give specifics of the incident pending the investigation.

Underwood also reminded boaters that vessels must slow to a “no-wake” speed when they are within 200 feet of displayed diver’s flags, and cannot approach within 100 feet of flags.

Vessels conducting scuba, snorkeling or free-diving activities may approach within 100 feet of a displayed flag at no-wake speed.

Boaters are also reminded that a 2014 rule requires anyone operating a power- driven boat with a 10-horsepower motor or larger to take a boater safety education class and carry Personal Watercraft Certification documents whenever they are operating a boat, Underwood said.